The invention relates generally to systems for liquefying cryogenic gases and more particularly, to a liquefaction system that produces liquid natural gas (LNG) by vaporizing liquid nitrogen or other cryogenic liquids where the resulting gas is directed to storage cylinders for use in welding and other applications.
LNG is one alternative energy source which is domestically available, environmentally safe: and plentiful when compared to oil. As a result, the use of LNG as a fuel for vehicles such as buses, trucks and the like has greatly increased. Entire fleets of government and industrial vehicles, as well as some privately owned vehicles, have been successfully converted to LNG power.
In addition, due to its clean burning qualities and convenience, natural gas has become widely used in a variety of applications, such as heating homes. Many sources of natural gas are located in remote areas, great distances from any commercial markets for the gas. Normally a pipeline is available for transporting the natural gas to commercial markets. When pipeline transportation of natural gas is not feasible, however, it is desirable to convert the natural gas into LNG for transport and storage purposes. The primary reason for this is that the liquefaction enables the volume of natural gas to be reduced by a factor of about 600. While the capital and running costs of the systems required to liquefy the natural gas are very high, they are still much less than the costs of transporting and storing unliquefied natural gas. In addition, it is much less hazardous to transport and store LNG than unliquefied natural gas.
Natural gas, which is primarily methane and obtained in gaseous form from gas and oil fields occurring in nature, is discharged from the earth to form a natural gas feed which requires processing before it can be used commercially. Natural gas may also be obtained from landfills and bio-waste. A typical natural gas stream enters an LNG plant at a pressure of approximately 600 psig to 1000 psig and a temperature of approximately 68xc2x0 F. to 104xc2x0 F. The raw natural gas feed enters an LNG plant and is processed through a variety of operations in different installations to finally emerge as LNG. The LNG is subsequently stored and transported to another suitable site for use. Sometimes, this use includes re-vaporization of the LNG.
During processing, the natural gas feed is first purified to remove or reduce :the concentrations of impurities or contaminants before it is cooled to form LNG. These contaminants may include, for example, carbon dioxide, hydrogen sulfide, mercury and water. Purification reduces the chances of blockage to the equipment used in the processing and avoids other processing difficulties.
Once all of the contaminants are removed from the natural gas feed, it undergoes cooling, to produce LNG. Conventional liquefaction cools the gas to a temperature of approximately xe2x88x92240xc2x0 F. to xe2x88x92260xc2x0 F. at or near atmospheric pressure. The refrigeration equipment that cools the natural gas represents a significant part of an LNG plant""s cost.
Many systems exist in the prior art for the liquefaction of natural gas by sequentially passing the gas at an elevated pressure through a number of cooling stages whereupon the gas is cooled to successively lower temperatures until the gas liquefies. Cooling is generally accomplished by heat exchange with one or more refrigerants that are arranged in a closed refrigeration cycle, such as the cascade cycle, the multi-component cycle, the expander cycle or the nitrogen cycle. The equipment for such cycles, however, includes compressors, expansion turbines and/or multiple special heat exchangers. As a result, the construction of such refrigeration systems is very expensive.
Alternative refrigeration systems use the vaporization of cold liquids, such as liquid nitrogen, to liquefy natural gas. Such systems often use a pressure vessel within which a recondensing coil is placed. The natural gas is directed to the pressure vessel and the coolant, liquid nitrogen for example, passes through the recondensing coil. As a result, the natural gas in the pressure vessel is liquefied and the nitrogen in the recondensing coil is vaporized. The gaseous nitrogen thus produced is used to regenerate the purification system that acts upon the natural gas feed that comes into the plant. The production of LNG in this fashion is relatively expensive in that approximately 1.6 gallons of nitrogen are used to produce one gallon of LNG.
There are a number of processes which require and consume gases such as nitrogen. An example of such a process is welding. Welding supply distributors routinely pump liquid nitrogen or argon at a high pressure through a vaporizer so that high pressure gas is produced. The high pressure gas is then directed to, and stored within, high pressure gas cylinders. The gas cylinders are then delivered to use points where welding operations are performed. The vaporizers warm the pressurized liquid flowing therethrough with ambient heat. As a result, the refrigeration or xe2x80x9cfree coldxe2x80x9d provided by the heat absorbed in vaporizing the nitrogen or argon is wasted. Other types of cryogenic liquids are vaporized in a similar fashion to produce gases for various processes or applications and thus also result in wasted refrigeration.
Accordingly, it is an object of the present invention to provide a natural gas liquefaction system that minimizes the amount of specialized treating equipment required.
It is another object of the present invention to provide a natural gas liquefaction system that minimizes the consumption of nitrogen or other cryogenic substances in the provision of refrigeration.
It is another object of the present invention to provide a natural gas liquefaction system that is economical to construct.
It is still another object of the present invention to provide a natural gas liquefaction system that is economical to operate.
It is still another object of the present invention to provide a natural gas liquefaction system that takes advantage of the free cold consumed during the vaporization of liquid nitrogen or other cryogenic liquids during the production of gases for use in other processes or applications.
Other objects and advantages will be apparent from the remaining portion of the specification.
The present invention is directed to a liquefaction system for liquefying a cryogenic gas, preferably natural gas, with free cold from a cryogenic liquid by vaporizing the cryogenic liquid, preferably liquid nitrogen. The system includes an insulated source tank containing a supply of liquid nitrogen with a head space there above filled with nitrogen vapor. The system also includes a heat exchanger having a gas inlet, a liquid outlet, a liquid inlet and a gas outlet. A purifier containing an absorbent receives a feed of natural gas and is in communication with the gas inlet of the heat exchanger. The heat exchanger liquid inlet is in communication with the source tank. A pump is in circuit between the source tank and the liquid inlet.
An insulated destination tank is in communication with the liquid outlet of the heat exchanger and a number of cylinders, or other process system, are in communication with the gas outlet of the heat exchanger. Liquid nitrogen from the source tank flows to the heat exchanger when the pump, or other means for transferring a cryogenic liquid, is activated while natural gas from the purifier simultaneously flows to the heat exchanger. As a result, the liquid nitrogen is vaporized in the heat exchanger by heat from the natural gas and the natural gas is liquified in the heat exchanger by free cold from the liquid nitrogen. The resulting nitrogen gas flows to the cylinders and the resulting LNG flows to the destination tank. A vaporizer is selectively in circuit between the pump and the cylinders so that when liquefaction is not taking place, the vaporizer may receive cryogenic liquid from the source tank and the pump and vaporize it so that nitrogen gas flows to the cylinders. An alternative type of cryogenic liquid may be substituted for the liquid nitrogen in the source tank so that an alternative type of gas is produced and flows to the cylinders.
Regeneration of the purifier between liquefaction cycles occurs in three stages. First, a vacuum pump pulls a vacuum on the purifier. Next, a valve is opened in a line that runs from the head space of the source tank to the purifier so that nitrogen vapor from the head space of the source tank travels to the purifier so that the absorbent therein is flushed with the nitrogen vapor. Finally, an air pump flushes the absorbent of the purifier with ambient air.